Diaryl methylphosphonates and methods for their preparation have long been known in the art. Inter alia, Michaelis et al., Ber. 31, 1048 (1898); Morgan et al., J. Am. Chem. Soc., 74, 4526 (1952); Landauer et al., J. Am. Chem. Soc., 1953, 2224; and Behrman et al., J. Org. Chem., 35, 3063 (1970) all disclose the preparation of diphenyl methylphosphonate. The prior art method involves the reaction of methyl iodide with triphenyl phosphite to form a phosphonium iodide intermediate which is subsequently decomposed by alcohols or aqueous cuastic to form diphenyl methylphosphonate. This earlier reaction is characterized by the use of a relatively large ratio (at least one molar equivalent) of costly methyl iodide and relatively low yields, generally less than 70%.
It is believed that these factors have reduced the commercial potential of diphenyl methylphosphonate and other diaryl methylphosphonates in spite of the fact that these compounds have long been known as useful intermediates in the preparation of various other compounds. For example, Coover et al., in U.S. Pat. No. 2,682,552, disclose the reaction of diphenyl methylphosphonate with dihydroxyaromatics to provide polymeric organophosphonates.
Related compounds have also been disclosed in the art wherein the methyl group is replaced by other aliphatic or aromatic groups. These related compounds are usually prepared through the employment of similar reactions. In German Offenlegungsschrift No. 2,158,765 (1971), ethylene glycol is reacted with triphenyl phosphite in the presence first of a catalytic amount of alkali metal alkoxide (formed from e.g. sodium metal plus glycol) and then a catalytic amount of an alkali metal iodide to form a bis phosphonate at temperatures in the range of 220.degree.-240.degree. C. However, as described in this application in detail, this process is done stepwise and phenol is first distilled off taking advantage of the high boiling glycol. It would be expected that this process would fail with an alcohol boiling well below phenol, and it is probably no oversight that the cited German reference is limited to glycols, all of which are high boiling.
In accordance with the present invention, it has been surprisingly found that diaryl methylphosphonates can be effectively prepared in high yield through the reaction of the corresponding triaryl phosphite with methanol in the presence of a catalytic amount of methyl iodide at a temperature from about 170.degree. C. to about 250.degree. C.
The process of this invention can be characterized by the following reaction diagram: ##STR1##
The term aryl as used herein and represented above as Ar-, is intended to include aryl radicals containing from 6 to about 14 carbon atoms inclusive. Illustrative of compounds included within this definition are phenyl, naphthyl, anthracyl, phenanthryl and the like.
Also included within this definition are aryl radicals which are substituted by non-interfering substituents such as alkyl, phenyl, chloro, bromo, alkoxy, aryloxy, alkylthio, arylthio, cyano, nitro and hydroxy.
By way of further illustrations of compounds falling within the definition of aryl, the following radicals are exemplary:
tolyl PA0 xylenyl PA0 chlorophenyl PA0 t-butylphenyl PA0 methoxyphenyl PA0 phenoxyphenyl PA0 phenylphenyl PA0 benzylphenyl PA0 methylthiophenyl PA0 isopropylthiophenyl PA0 cyanophenyl PA0 nitrophenyl PA0 mesityl.
The term "catalytic amount" as used herein means from about 0.1 to about 10% by weight based on the triaryl phosphite. Amounts from about 0.5 to 5% by weight are generally employed.
The reactants can be employed in stoichiometric amounts or in a ratio of from about 2:1 to 1:2 parts triaryl phosphite to methanol. For best yields, however, it is preferred to employ methanol in slight excess over stoichiometric.
Temperatures employed in this reaction are from about 150.degree. to 300.degree. C. While superatmospheric pressures can be employed in the reaction of the present invention, it can be effectively conducted at atmospheric pressure. This appears to be quite unexpected in view of the volatility of methanol and the reaction temperatures employed.
It has been additionally found that diphenyl methylphosphonate can be reacted with aliphatic polyols to provide useful and in some instances novel compositions.
Suitable polyols include pentaerythritol, di(pentaerythritol), neopentylene glycol, trimethylolpropane, di(trimethylolpropane), glycerine, and the like.
These derivatives can be prepared by heating the reactants at a temperature of from about 170.degree. C. to about 250.degree. C. in the presence of a suitable catalyst such as magnesium chloride.
These compositions, all of which are useful as flame retardants for polymers can be represented by the following formulas: ##STR2## wherein n=1 to 10, and the chain can be straight or branched. ##STR3##
It will be appreciated that compounds II-IV and resins V-VI above are generally prepared in admixture, by reacting diphenyl methylphosphonate with pentaerythritol in various ratios. Analogous mixtures are prepared using dipentaerythritol or mixtures thereof with pentaerythritol. These phosphonate products, as mentioned, are flame retardants for plastics, textiles, and coatings. They also show surprising properties as smoke suppressant agents in these substrates.